Flaw detection method using a strippable acrylic resin aerosol fixer



United States Patent US. Cl. 252-408 4 Claims ABSTRACT OF THE DISCLOSURE A pressurized aerosol type of non-flammable liquid composition for use in fixing an already developed color indication on the surface of a workpiece that has been treated with a colored liquid penetrant for the detection of surface discontinuities. The fixer compositon is a solution in a common solvent of an-acrylic type of film-forming resin, which is not readily strippable, and a liquid release agent, such as modified phthalic glycerol alkyd resins, petroleum lubricating oils and long chain fatty oils and fatty acids. The release agent, while miscible with the film-forming resin in the common non-flammable solvent, tends to become immiscible as the solvent evaporates and to form an oil surface on the underside of the film as it dries out on the surface undergoing test, thereby imparting to the dried film the property of being easily stripped.

DETAILED DESCRIPTION This invention relates to a strippable aerosol fixer and more particularly to a pressurized aerosol composition containing a film-forming resin and a liquid release agent dissolved in a common solvent that is nonflammable, together with a compatible liquefied normally gaseous propellant.

It has heretofore been known to use a film-forming resin or plastic lacquer as a developer in the penetrant method for the detection of surface discontinuities in a workpiece, and thereafter to strip the resulting dried film containing the color indication in order to make a record thereof.

The present invention has for some of its objects to improve the strippable qualities of the dried film; to facilitate the application of the film-forming composition to the workpiece by making the composition available in the form of an aerosol; to render the composition substantially nonfiammable by the use of nonflammable solvents; and to provide a composition particularly adapted for use as a fixer for an already developed color indication.

In the present-day method of inspection by the penetrant technique, a liquid penetrant is applied to the cleaned surface of the workpiece to be tested. The liquid penetrant may comprise, as the liquid vehicle, either a volatile or relatively non-volatile solvent for the dye contained in the penetrant. The dye can be either a visible dye and the inspection made under white light, or a fluorescent dye and the inspection made under near ultraviolet or black light. After the application of the liquid penetrant, time is allowed for the penetrant to penetrate into any surface flaws, and then the excess of penetrant on the surface is removed while leaving any entrapped penetrant or penetrant residue in any flaws that may be present. In the next step, a developer is applied, either as a wet solvent type or as a dry powder type developer, and the inspection made under proper light source.

Where a resin or plastic solution or lacquer is used as the wet developer, it is allowed to dry by evaporation of Patented Dec. 8, 1970 the solvent to leave on the surface of the workpiece a dried film that contains the developed color indication, and that may or may not be strippable.

In the preferred use of my aerosol type composition, the composition is applied as an aerosol to an already developed color indication in order to fix the indication, that is, to preserve the indication without any substantial spreading or blurring of the color indication so as to leave the color indication as finely defined and distinctly delineated as possible in high fidelity relation to the actual size and dimensions of the surface discontinuity, or crack. The result is a high resolution of the defect even in the case of closely spaced, extremely minute cracks.

In order to prevent, or substantially reduce, the tendency of the color indication to spread upon application of my fixer composition, the application thereof as an aerosol is accomplished in a plurality of substantially dry passes, namely with the nozzle of the aerosol can at such a distance from the surface undergoing test that the aerosol spray or mist is substantially dry as it hits the color indication. If the spray were too wet at the time of impact, the color indication would be redissolved by the solvent of the fixer, but by keeping the spray relatively dry when it hits the substrate, there is no excessive dis solution and therefore no bleeding and spreading of the color indication. Yet the applied fixer has sufficient adhesiveness toward the substrate to adhere to and unite with the color indication, and sufiicient cohesiveness, after drying, to form a readily strippable film that includes on its undersurface (or body) the color indication. This can be accomplished whether the indication has been developed by a dry powder, such as talc, or by a wet solvent developer, such as a lacquer, with or without a pigment suspended therein, or even if the color indication has not been previously developed. In this latter case, the aerosol composition would function as a developer of the indication, and also as a fixer, but in order so to function the initial passes should be more on the wet side in order to dissolve the dye out of the crack or defect, and the resulting film or films should be allowed to dry between passes, so that the final composite dried film is built up to a thickness of the order of a few mils or to a sulficient thickness that it is cohesive enough to be strippable as a self-supporting, unitary film of the desired size.

In the use of my fixer composition it is preferable to apply the aerosol to a penetrant treated surface after development of a color indication by means of a pigmented solvent type developer. Such a developer may comprise a sprayable or an aerosol type of liquid composition comprising a highly volatile solvent that is a solvent for the penetrant and/or the penetrant residue entrapped in any surface defect, and including a finely divided pigment, such as titanium dioxide, a pyrogenic silica or other lightcolored mineral substance, dispersed in the solvent. Upon being applied to the surface, the solvent dissolves the entrapped colored penetrant or penetrant residue and dye therein to form a color indication at the site of the defect and, upon evaporation of the solvent, the pigment that remains on the surface functions both to develop the color indication by absorption, adsorption and/or capillary action, and also to provide a light-colored contrasting background for enhancing the visibility of the developed color indication. A solvent type of pigmented wet developer such as is preferably used ahead of the application of my fixer contains no dissolved resin or plastic, so leaves a layer of more or less discrete particles of pigment lightly adhering to the substrate, or surface, of the workpiece. applied, it forms a resinous film over the pigment particles, including the colored particles constituting the color indication, and unites or embeds such particles into the resulting dried and strippable film.

My fixer composition comprises, as its active components, a film-forming acrylic resin and a liquid release agent that functions to facilitate the stripping of the resinous film just referred to. By the term acrylic resin is meant those polymers of acrylic acid, methacrylic acid and esters thereof that are commonly referred to as acrylic resins (see Modern Plastics Encyclopedia, 1967, p. 115). In preparing such a fixed composition, the film-forming acrylic resin and the liquid release agent are dissolved in a non-flammable, volatile solvent that is a common solvent for both and that is also miscible with the liquefied normally gaseous propellant that is used to convert the composition into an aerosol upon release from a conventional pressurized aerosol can containing the entire composition."'

The liquid release agents that have been found satisfactory for use in my aerosol fixer composition are modified alkyd resins, such as modified phthalic glycerol alkyd resins, petroleum base lubricating oils and high molecular weight (C -C fatty acids and fatty oils, such as castor oil, dehydrated castor oil and the fatty acids thereof known as 911 acids. These release agents are soluble in the solvent (termed a common solvent) in which the film-forming acrylic acid is dissolved and are miscible with such acrylic resins in the presence of the common solvent. However, during drying, or after drying of the film formed by the application of my composition as an aerosol to a surface, the release agent appears to migrate to the undersurface of the film and impart to such surface an oily or greasy feel. It is believed that this phenomenon is responsible for the release action that results, but whatever the cause, the use of such release agents as described above renders the film much more readily strippable from any surface to which may composition has been applied.

The common solvent used in my composition is preferably non-fiammable and for this purpose highly volatile. Chlorinated hydrocarbon solvents are best suited, such as trichloroethylene, perchloroethylene, 1,1,1-trichloroethane, ethylene dichloride, or mixtures thereof. The common solvent is present in such a major proportion that the composition as a whole is relatively nonfiammable even though other solvents, including flammable solvents, are brought into the composition along with the acrylic resin or release agent.

As the liquefied normally gaseous propellant, I prefer to use a nonflammable fiuorohydrocarbon such as Genetron 12, also referred to as F-12, which is dichlorodifluoromethane. However, other nonfiammable propellants can be employed.

In making up an aerosol composition for dispensing from a conventional 12 oz. can, the following is a typical formula for the base liquid, viz the composition before adding the propellant:

150 grams Acryloidl3-44 100% solids) 900 ml. trichloroethylene 35 ml. Triton B-1956 (77% solids) The Acryloid B44 resin is first dissolved at room temperature in the trichloroethylene to form a stable solution, and the Triton B1956 then added and dissolved in the solution with agitation. After the solution has been prepared, 150 ml. there are packaged, along with 172 grams of the propellant, in a conventional 12 oz. aerosol can. The can is then ready for use in dispensing the contents thereof as an aerosol.

The physical properties of the aerosol composition are as follows:

Specific gravity at 20 C.-1.457 grams ml. Viscosity (average at 24.5 C.)1921 centistokes. No flash point.

Toxicity-no greater than that of the trichloroethylene.

4 The following table gives proportions by weight of the various ingredients of my fixer composition, prior to packaging with a propellant:

TABLE [Percentages by weight of ingredients] Minimum Preferred Maximum Formula No. 1:

B-44 resin 1 7. 0 12. 2 14. 3 91. 4 84. 5 74. 5 1. 6 3. 3 11. 2

Formulas Nos. 2, 3, and 4, same as Formula No. 1 but in place of B-1956 resin, the same percentages of the following release agents are used, viz:

Formuia No. 2, 9-11 acids;; Formula No. 3, Carneal oil 29.- Formula No. 4, Castor oil Formula No. 5:

A-lOl 3 'Irichloroethylene. B-1956 resin Formulas Nos. 6, 7 and 8, same as Formula No. 5 but in place of B- resin, the same percentages of the following release agents are used, v

Formula No. 6, 9-11 acids 0. 8 Formula No. 7, Carneal oil 29 Formula No. 8, Castor oil Formula No. 9:

A-2l-LV resin 4 4 Trichloroethyleue 4 13-1956 resin 0.6 0.

Formulas Nos. 10, 11 and 12, same as Formula No. 9 but in place of 13-1g56 resin, the same percentages of the following release agents are use viz:

Formula No. 10, 9-11 acids 0. Formula N0. 11, Carneal oil 29 0. Formula No. 12, Castor 011 0.

1 100% solids. 2 77% solids; 23% ethylene dichloride. 3 40% solids; 60% methylethyl ketone: b t30%lsolids; 70% solvents: 50% toluene; 40% methylethyl ketone; 10%

u ano Physical constants:

Solids 100%. Weight per gallon 9.6 lbs. Viscosity (40% solids in toluol/ methyl Cellosolve /5) 855 to 1700 cps. Appearance of solution Colorless to light yellow clear solution.

B-l956 is a liquid of low viscosity containing 77% by weight of a long oil modified phthalic glycerol alkyd resin dissolved in ethylene dichloride, and has the following physical and chemical properties:

Colon-Amber Odor-Mild agreeable odor Viscosity245 S.U.S. at F.

Specific gravity1.055 at 25 C./25 C. Average (8.8

lbs./ gallon) So1ubilityTriton B1956 is soluble in vegetable oils,

aromatic petroleum derivative solvents, aliphatic solvents and certain other solvents. It is relatively insoluble in water but Will form dispersions at low levels of concentration. It is used as a dispersing agent.

941 acids is a proprietary composition of 9,11 and 9,12 octadecadienoic acids, having a molecular weight of 281. It is a clear, bright, low viscosity liquid having the following properties:

Physical properties:

Density at 25 C., lb./gal 7.51 Sp. gr. at 25 C./25 C. .903 Ref. index at 25 C 1.4725

Color, Gardner 1- Volatile matter, percent 0.1 Pour point, F Flash point, COC, F 375 Fire point, COC, F. 425 Boiling point at 1 mm., C. 220 Viscosity, stokes at C 0.3

Chemical Properties:

Acid value 198 Iodine value (Wijs) 153 Total iodine value 180 Saponification value 199 Hydroxyl value 7 Conjugated dienoic acids, percent 33 Solubility at 25 C.:

Insoluble in water Soluble in most organic solvents.

Carneal Oil 29 is a lubricating oil, and is primarily an aliphatic petroleum product.

A-21LV resin is a low viscosity methylmethacrylate polymer in a 30% solution, having the following physical constants:

Solids 30%i0.5%. Solvent Toluene 50%. Methyl ethyl ketone Butanol 10%. Viscosity (cps. at 25 C.) 210 to 280 (29% solids). Color Colorless and clear. Weight per gallon 7.7 lbs.

A-lOl resin is a proprietary product available as Acryloid A-101, as a 40% solids content in methyl ethyl ketone. I have found that it produces a film, as used in Formula No. 5, that is free from blue fluorescence when inspected under black light. This makes Formula No. 5 particularly suited where a fluorescent dye is used in the colored liquid penetrant.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

I claim:

1. In a method for the detection of surface discontinuities in a workpiece that includes the application of an acrylic resin composition to a workpiece surface having a colored penetrant residue entrapped therein, the improvement whereby said acrylic resin composition is caused to form a readily strippab le film including therein the surface entrapped penetrant residue, said improvement comprising incorporating into said acrylic resin composition:

(1) a liquefied normally gaseous propellant;

(2) a release agent; and (3) a volatile solvent having common solvent properties toward said acrylic resin and said release agent to dissolve the same and being miscible with said propellant while in liquid form, said release agent becoming less miscible with said acrylic resin as said common solvent evaporates and thereupon imparting to the resulting dried film the property of being readily strippable from said surface, said release agent being selected from the group consisting of long oil modified phthalic glycerol alkyd resins, petroleum base lubricating oils and high molecular weigh fatty acids and fatty oils.

2. The method as defined by claim 1, wherein said volatile solvent is pigmented but resin-free.

3. The method as defined by claim 1, wherein said common solvent is nonflammable.

4. The method as defined by claim 1, wherein said acrylic resin and said liquid release agent are in the percentages by weight of from 7 to 14.3 and from 1.6 to 11.2%, respectively, of said aerosol composition.

References Cited UNITED STATES PATENTS 2,441,227 5/1948 Pineles 117-6 2,659,704 11/1953 Kerr 260-23 2,909,434 10/1959 Patten etal 1176 3,034,915 5/1962 Kornbluth 117 6 3,184,596 5/1965 Alburger 252 301.2

OTHER REFERENCES Paint Industry Technical Yearbook and Materials Manual, volume IV, 1959, Heckel Publishing Company, Philadelphia, Pa. pp. 305.

DONALD E. CZAJ A, Primary Examiner R. W. GRIFFIN, Assistant Examiner 1 US. Cl. X.IR.

73-104; ll7-6; 25071; 252--30l.2; 260-22, 23, 33.6

UNITED STATES 'PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 54 130 Dated Dec. 8, 1970 Inventor(s) Christie S. Mggdalin It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, lines 68 et seq. insert -Consequent1y, when my aerosol fixer composition is--.

Column 3, line 8, cancel "fixed", and substitute -fixer--.

Signed and sealed this 3rd day of August 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

